Diarylamine antioxidants are known and have been widely used to improve the thermal-oxidative stability and/or light induced degradation in numerous products used in engineering; for example, they can improve the performance properties in lubricants, hydraulic fluids, metal working fluids, fuels or polymers, just to name a few.
Commonly, these diarylamines have been alkylated, see for example, U.S. Pat. No. 2,943,112 which discloses an improved process for alkylating diphenylamine and U.S. Pat. No. 3,655,559 which discloses alkylated diphenylamines as stabilizers. Alkaryl substituted diphenylamines and phenylnapthylamines (such as α-methylstyryl-diphenylamine) are disclosed for example in U.S. Pat. Nos. 3,533,992; 3,452,056 and 3,660,290. Electron withdrawing groups attached to one or more of the phenyl rings has been found to diminish or completely destroy the inhibition ability of the base diphenylamine. Berger et al. Inhibition of Hydrocarbon Autoxidation, Developments in Polymer Stabilization-6, Elsevier Applied Science Publishers, 1983, pp 1-27, demonstrated that in a paraffinic oil (Shell Ondina 33, a mixture of high-boiling cycloalkanes and isoalkanes) at 130° C. the addition of nitro groups to a diphenylamine (inhibition coefficient of forty-one) completely destroyed the catalytic activity with a resulting inhibition coefficient to zero for 4-nitro-diphenylamine and bis-(4-nitro)-phenyl-amine. The authors explained that the unpaired electron density in the diarylnitroxyls was strongly de-localized as shown by electron spin resonance spectroscopic studies, and that as a result radical reaction would occur at the nitroxyl function but also undesirably at the aromatic rings. The authors concluded that under conditions of auto-oxidation the intermediate nitroxyl would he attacked by peroxyl radicals on the aromatic ring, eventually resulting in destruction of the inhibitor.
Additionally, alkyl substituted 1,2-dihydroquinoline and polymers thereof, have been employed as antioxidants, see U.S. Pat. Nos. 3,910,918. While, U.S. Pat. No. 5,310,491 discloses the reaction product of an alkyl substituted 1,2-dihydroquinoline with a diarylamine. Tetrahydroquinones and substituted tetrahydroquinones have also have also been disclosed as antioxidants, see for example U.S. Pat. Nos. 2,794,020; 3,362,929; 4,692,258 and 4,965,006; while U.S. Pat. No. 4,848,741 discloses certain N-substituted tetrahydroquinolines. Likewise decahydroquinolines and substituted decahydroquinolines have been employed as antioxidants, see U.S. Pat. Nos. 2,998,468 and 4,069,195.
Synergist and antagonist combinations of antioxidants have been disclosed. Effective synergistic mixtures of antioxidants are typically compounds that intercept oxidation by two different mechanisms. For example, those in which one compounds functions as decomposer of peroxides and the other compound functions as an inhibitor of free radicals. Well known heterosynergism has been disclosed between sulfur and phosphorous containing compounds (such as sulfides, dithiocarbamates, phosphites and dithiophosphates) and aminic or phenolic antioxidants. U.S. Pat. No. 2,718,501 discloses a synergistic mixture of a sulfur-containing compound, such as a wax sulfide or dioctadecyl disulfide, and an aromatic amine compound having at least 2 aromatic rings, such as phenyl alpha-naphthyl amine, for use in preventing oxidation in lubricating oils. For example, U.S. Pat. No. 2,958,663 discloses an extreme pressure lubricant composition containing from 0.01 to 5 percent each of sulforized oleic acid, C18-C22 alkenyl succinic acid, chlorinated paraffin wax containing from 20 to 60 percent chlorine, diphenylamine and N,N-salicylal-1,2-propylenediamine. U.S. Pat. No. 3,345,292 discloses stabilized alkyl substituted diaryl sulfides for use as functional fluids where the stabilizer can be diaryl amine or alkylated phenol, U.S. Pat. No. 4,089,792 discloses lubricants having an antioxidant mixture of a primary amine and an antioxidant selected from aromatic or alkyl sulfides and polysulfides, sulforized olefins, sulforized carboxylic acid esters and sulfurized ester-olefins.